Two-Axis Solar Concentrator System

ABSTRACT

A system for use on a surface to collect solar energy from the sun has a stand, a module, and solar collector(s). The stand supportable on the surface has rotational points rotatably supporting the module so it can rotate about a first axis of rotation. A first drive disposed on the stand is operable to provide first rotation, and a cable connected between a hoop pulley of the module and the first drive on the stand can rotate the module about the first axis to direct the solar collector(s) toward the sun. The solar collector(s) disposed on the module can be photovoltaic cells for collecting solar energy. A second drive on the module can rotate an adjacent solar collectors on the module using pulleys and cable. Reflectors on the collectors can focus the sun rays to photovoltaic cells. The second drive can rotate the collectors about a second axis, carried by the first axis, to direct the solar collector(s) toward the sun.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.15/382,957, “Two-Axis Solar Concentrator System,” filed Dec. 19, 2016,which claims the benefit of U.S. Provisional Patent Application Ser. No.62/270,305, filed Dec. 21, 2015.

FIELD OF THE DISCLOSURE

The subject matter of the present disclosure relates to solar panelsused to generate electrical or thermal power. More particularly, thesubject matter relates to solar panels comprising an array of solarconcentrators with optical gain in two axes utilizing photovoltaic cellsto generate electricity.

BACKGROUND OF THE DISCLOSURE

Concentrators for solar energy have been in use for many years. Thesedevices are used to focus the sun's energy into a small area to raisethe power level being concentrated on a photovoltaic converter togenerate electrical power directly, or on a fluid line to heat water tomake steam to drive a turbine to generate electrical power.

One difficulty with these concentrators has been that they are generallylarge and bulky and are not suitable for residential applications orother locations where the aesthetics of the installation are ofimportance. Additionally, they are very susceptible to environmentaldamage due to wind and other elements.

Thin film solar panels have been used extensively in recent years forinstallations on buildings and homes. These panels may be articulated inone or two dimensions or may be fixed. A fixed installation is the leastexpensive implementation but is also the least efficient because theplane of the panel is rarely normal (90°) to the solar axis. A singlepoint of articulation that allows a panel to track the angle of the sunduring the day improves on this approach but there will still be anangle. A second, orthogonal point of articulation will allow a panel totrack the sun and maintain the panel so that the solar axis is normal tothe plane of the panel. Such systems are inherently bulky and in generalare considered more suitable for surface installations than for roof topinstallations.

An additional problem with roof top installations of any type is theproblem of shading. Shading often occurs by environmental features suchas trees or tall buildings nearby. Little or nothing can be done aboutthese other than using good planning and commonly available softwaretools. The difficulty that shading poses is that it can also affect theoutput of portions of a solar installation that are not placed in theshading because of electrical interaction between the shaded areas andthe unshaded areas of the solar installation. This phenomenon isunderstood and is well reported in the literature. “Shading Effects onOutput Power of Grid Connect Photovoltaic Generator Systems,” Hanitschet al, Rev. Energ. Ren.: Power Engineering (2001) 93-99, providesenlightening information on the matter.

Another cause of shading relates to installation factors. Individualsolar concentrator assemblies may shade adjacent solar concentratorassemblies at times of the day, with sunrise and/or sunset being themost frequent times. Solutions to these problems depend on the type ofsolar system involved. A solution for concentrating solar energy systemsis particularly difficult. A consideration of solutions is presented inthe present application

FIG. 1 presents a prior art solution for the need to keep the opticalaxes of a plurality of trough mirrors aligned while moving the troughmirrors to maintain alignment with the sun. Articulating solarconcentrator system 10 comprises trough mirrors 25, solar energycollecting tubes 40, support 30, energy transmitting tube 35, troughmirror linking rod 20 and linking rod actuator 15. Trough mirrors 25 arearrayed parallel to one another with solar energy collecting tubes 40 atthe focal point of the trough mirrors 25. Solar energy collecting tubes40 are each physically connected to one of the trough mirrors 25 and aresupported by mounting assemblies 30 such that all energy collectingtubes 40 lie in the same plane. Energy transmitting tube 35 connects toeach collecting tube 40 by a sealed fitting or the like as is well knownin the art. Energy transmitting tube 35 is supported by energycollecting tubes 40. Energy transmitting tube may be connected to asuitable energy harvesting system (not shown) such as a turbine or a hotwater heater as is well known in the art. Linking rod 20 is connectingto each of the trough mirrors 25 by an articulating component (notshown). When linking rod 20 is moved by linking rod actuator 15 alltrough mirror respond by rotating in the same direction. In thisimplementation the movement of trough mirrors 25 enables solarconcentrator system 10 to track the sun during the course of a day.

Wire cables have long been used as a way of transferring mechanicalenergy from one location to another. Wire cables are used in diverseapplications such as moving the control surfaces of aircraft and movingthe print head of dot matrix printers.

One example is the Texas Instruments TI810BSC, designed by the inventorof the present disclosure, which was used for an extended period of timein the airline industry to print tickets and baggage tags. The printeruses a driver pulley to move the print head back and forth across thepaper or other material to be printed by moving a wire cable. The wirecable loops over an idler pulley and is routed back to the driverpulley. A tensioner pulley is typically added in the return path of thewire cable. The interior of a dot matrix printer is a hostileenvironment because in decades of service a significant amount of paperlint and dust will accumulate there. The wire cable drive arrangementswork well in that environment and also return the print head to aprecise position.

Many of these wire cables are pre-stretched to minimize the developmentof additional slack in the wire cable during extended operation. Theslack occurs as a natural consequence of the catenary effects of gravityon any cable suspended from two ends. The slack follows the shape of thehyperbolic cosine function. The slack becomes problematic as the driverpulley reverses its direction of motion. Too much slack can lead to anunacceptable level of play.

The present disclosure seeks to overcome the foregoing difficultiesassociated with rooftop installations while maintaining the ability totrack the sun in two axes through the use of pulleys driven by wirecables.

The subject matter of the present disclosure is directed to overcoming,or at least reducing the effects of, one or more of the problems setforth above.

SUMMARY OF THE DISCLOSURE

A solar energy concentrator in the form of a plurality of articulatingsolar concentrator modules mounted parallel within a frame receivessolar energy into each module and focuses it onto a series ofphotovoltaic cells within each module that convert the solar energy intoelectrical energy. The articulating solar concentrator modules are movedby a pulley arrangement configured such that a driver pulley moves awire cable that is routed to each driven pulley causing the drivenpulleys to move the solar concentrator modules in concert. The frame ismounted to a stand by two axles oriented coaxially and orthogonal to theorientation of the solar concentrator modules. A hoop pulley attached tothe frame parallel to the articulating solar concentrator modules ismoved by a wire cable moved by a motor attached to the stand.

The solar concentrator modules of the present disclosure can include aplurality of spherical or aspheric half mirrors arrayed so as to focussolar energy on a plurality of photovoltaic cells arranged in a circuitto gather electricity generated by the photovoltaic cells and deliverthe electricity to a circuit on the frame. Alternatively, the solarconcentrator modules of the present disclosure can include a pluralityof Fresnel lenses arrayed so as to focus solar energy on a plurality ofphotovoltaic cells arranged in a circuit to gather electricity generatedby the photovoltaic cells and deliver the electricity to a circuit onthe frame.

A system for tracking the sun in two axes optics using wire cablepulleys to enable the use of high gain solar concentrator is described.

The foregoing summary is not intended to summarize each potentialembodiment or every aspect of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a prior art implementation of a system to keep a plurality ofconcentrator modules pointed at the same angle relative to a frame.

FIG. 2A depicts a perspective view of a solar concentrator systemaccording to the present disclosure.

FIG. 2B depicts a perspective view of another configuration of the solarconcentrator system.

FIG. 3A depicts a perspective view of a solar concentrator module.

FIG. 3B depicts an expanded view of the components of the solarconcentrator module.

FIG. 3C depicts a ray trace of a reflective for the solar concentratormodule.

FIG. 4A depicts a drive (clamp) mount end of the solar concentratormodule.

FIG. 4B depicts an electrical connector (axle) end of the solarconcentrator module.

FIG. 4C depicts a pulley, a bearing housing, and axle combinationcompatible with the drive mount end of the solar concentrator module.

FIG. 4D depicts a bearing housing assembly compatible with the axle endof the solar concentrator module.

FIGS. 5A-5B depicts other perspective views of the disclose system.

FIG. 5C depicts an end view of a hoop pulley and driver motor for thedisclosed system.

FIG. 5D depicts an arrangement of the rotations points between the standand frame.

FIG. 5E depicts one way the one or more wire cables can connect to adrive pulley for the hoop pulley.

FIGS. 6A-1 and 6A-2 depict front and side views of a first configurationfor tensioning the hoop pulley drive system by an extension spring.

FIG. 6B depicts a second configuration for tensioning the hoop pulleydrive system using a tensioning pulley mounting outside the drive pathof the hoop pulley drive system.

FIG. 6C depicts a third configuration for tensioning the hoop pulleydrive system using an extension spring mounted at the end of the hoop.

FIG. 6D depicts a fourth configuration for tensioning the hoop pulleydrive system using an in-line extension spring positioned between thetwo ends of the hoop.

FIG. 6E depicts a method of mounting a wire cable to a hoop pulley.

FIG. 6F depicts a driver pulley configured for a hoop pulley system.

FIG. 7A depicts a perspective view of a frame of the disclosed solarconcentrator system.

FIG. 7B depicts a perspective view of a partially assembled frame of thedisclosed solar concentrator system.

FIG. 7C depicts a seasonal driver motor affixed to the frame of thesystem.

FIG. 7D depicts two driven pulleys and a driver pulley without wirecable.

FIGS. 7E-7F depict a pulley wherein a wire cable is held in place on thepulley by a clamp having a pan head screw.

FIG. 7G depicts a pulley drive system comprising a linear stepper motor.

FIG. 7H depicts details of a linear stepper motor.

FIG. 7I depicts a shaft of a linear stepper motor inserted into ananti-rotational sleeve.

FIG. 8A depicts a tensioner having an extension spring in-line with thereturn side of the wire cable.

FIG. 8B depicts a tensioner having a turnbuckle placed in-line with thereturn side of the wire cable;

FIGS. 8C-1, 8C-2, and 8C-3 depict a side view, top view, and cutawayview of a pulley system with a tensioner having a tensioning pulley andan extension spring.

FIG. 8D depicts a pulley system having a tensioner with an in-lineextension spring between an idler pulley and the driver pulley.

FIG. 8E depicts a pulley system having a tensioner with a tensionerpulley interfaced to the driver and driven pulleys through a drivenpulley.

FIG. 5F depicts a pulley system having a tensioner with a tensionerpulley system and a compression spring.

FIG. 8G depicts a pulley (driver or driven) which is wrapped by a wirecable extending to a tensioner pulley and returning from the tensionerpulley.

FIG. 8H depicts a pulley system in which a portion of a wire cable isreplaced by a tensioner.

FIG. 8I depicts a cable loop formed by a cable and ferrule.

FIG. 8J depicts a tensioner comprising a turnbuckle and two rods.

FIG. 8K depicts an eyelet bolt suitable to support a tensioner rod.

FIG. 8L depicts an autonomous tensioner assembly.

FIG. 8M depicts an alternative autonomous tensioner assembly.

FIGS. 9A-9C respectively depict perspective, side, and end view of asolar concentrator module having a brush attached on the underside.

FIGS. 10A-1, 10A-2, and 10A-3 depict a side view, a top view, and acutaway view of a pulley system having a tensioner with a tensioningpulley and an extension spring, and having driven pulleys wrapped bywire cable on both the drive side and the return side of the wire cable.

FIG. 10B depicts a pulley system having a tensioner with an in-lineextension spring between an idler pulley and the driver pulley andhaving the driven pulleys wrapped by wire cable on both the drive sideand the return side of the wire cable.

FIG. 10C depicts a pulley system having a tensioner with a tensionerpulley interfaced to the driver and driven pulleys through a drivenpulley and having the driven pulleys wrapped by wire cable on both thedrive side and the return side of the wire cable.

FIG. 10D depicts a pulley system having a tensioner with a pulley and acompression spring and having the driven pulleys wrapped by wire cableon both the drive side and the return side of the wire cable.

FIG. 11A depicts a set of driven pulleys connected by wire cable whereinthe wire cable is held secure to each of the driven pulleys by afastener;

FIG. 11B depicts a set of driven pulleys connect by a wire cable heldsecure by a fastener wherein one driven pulley is the last pulley in aseries of driven pulleys;

FIG. 11C depicts a last pulley in a series of driven pulleys wherein arange of positions for fastening is indicated;

FIG. 11D depicts an alternate view of a last driven pulley in a seriesof driven pulleys;

FIG. 11E depicts a first driven pulley in a series of driven pulleys anda driver pulley connected by wire cable such that the driver pulley mayturn all driven pulleys together;

FIG. 11F shows another way to fasten cable to a pulley;

FIG. 12A depicts a rooftop solar concentrator installation wherein aside of the roof is oriented north-south;

FIG. 12B depicts a rooftop solar concentrator installation wherein noside of the roof is oriented north-south;

FIG. 12C depicts a side view of a solar concentrator frame wherein oneend of the solar concentrator frame is at a different height from theopposite end of the solar concentrator frame;

FIG. 13A depicts four sectors for the angles of the daily and seasonaldrive systems;

FIG. 13B depicts a configuration of a rectangular frame wherein theframe is parallel to the ground;

FIG. 13C depicts a configuration of a rectangular frame wherein twosides of the frame are not parallel to the ground;

FIG. 14A presents a convention for assignment of positive and negativeangles to describe the orientation of the daily axis relative to thelocal ground plane;

FIG. 14B presents a convention for assignment of positive and negativeangles to describe the orientation of the daily axis relative to truenorth;

FIG. 14C presents a convention for depiction of the east-west axis andaltitude axis of a solar concentrator system;

FIG. 15A presents a 3D Cartesian depiction of a coordinatetransformation of solar azimuth and solar altitude to the localcoordinate system of a solar concentrator system of arbitraryorientation;

FIG. 15B presents a planar Cartesian view of the coordinatetransformation of solar altitude and solar azimuth to the localcoordinate system of a solar concentrator system of arbitraryorientation;

FIG. 15C presents the calculation elements of an algorithm operative totransform solar azimuth and solar altitude to the local coordinatesystem of a solar concentrator system of arbitrary orientation;

FIG. 16A presents the reflectance of an aluminum mirror across a rangeof wavelengths from 250 nm to 2500 nm.

FIG. 16B presents a mirror substrate with curvature in two dimensions oraxes.

FIG. 16C presents a mirror substrate with curvature dimensions and aplanar film to be applied to the substrate.

FIG. 16D presents a pattern of slots defined in a planar film to enableit to be applied to a mirror substrate with curvature in two dimensions

FIG. 16E presents a vacu-forming device utilizing air pressure outside avacuum area to force compliance of a first material onto a secondmaterial within the vacu-forming device.

FIG. 16F presents an adaptation of the vacu-forming device of FIG. 16Ewith the addition of a deformable, spherical pressure medium locatedoutside the vacuum area.

FIG. 16G depicts a process of providing for external pressure on acurved surface within a vacu-forming device.

FIG. 16H depicts a perspective view of the stack for the process ofproviding pressure on a curved substrate.

FIG. 16I depicts a side view of a process for providing pressure on acurved substrate.

DETAILED DESCRIPTION

FIG. 2A depicts a solar concentrator system 50 according to the presentdisclosure for use on a surface to collect solar energy from the sun.The system 50 includes a base or stand 70, a first drive, a frame ormodule 80, and one or more solar collectors 55. The stand 70 issupportable on the surface (not shown), such as the ground, a rooftop,or the like. The stand 70 has first opposing ends 74, which includefirst rotational points 75. The module 80 has second opposing ends 84with second rotational points 85. As assembled, the second rotationalpoints 85 of the module 80 rotatably connect to the first rotationalpoints 75 of the stand 70. In this way, the module 80 is balanced androtatable on the first and second rotational points 75, 85 about a firstaxis A₁ of rotation.

To rotate the module 80, the system 50 uses the first drive and a hooppulley 60. The first drive is disposed on the stand 70 and is operableto provide first rotation. The hoop pulley 60 disposed on the module 80defines a curvature about the first axis A₁, and at least one firstcable (not shown) connected between the hoop pulley 60 and the firstdrive is used for rotating the module 80 about the first axis A₁ inresponse to the first rotation of the first drive. Rotation of themodule 80 about the first axis A₁ enables the planar face of the module80 having one or more solar collectors 55 disposed thereon to collectsolar energy of the sun as it traverses across the sky.

As shown in particular in FIG. 2A, the stand or base unit 70 with thefirst drive includes a daily actuator or drive motor 65 and dailyrotational points 75 (axels, axel mounts, bearings, slots, etc.) affixedthereto. The frame 80 has the hoop pulley 60 affixed thereto forconnection to the daily drive motor 65. The frame 80 is mounted with itsrotational points 85 (axels, axel mounts, bearings, slots, etc.) ondaily rotational points 75 of the stand 70 for rotation thereabout asnoted above.

The module or frame 80 can have one or more solar collectors 55, such asa conventional solar panel having a number of photovoltaic cells (notshown) disposed on a planar surface (not shown) supported on the frame80. Preferably, and as shown in FIG. 2A, the frame 80 has a plurality ofsolar collectors 55 arranged to articulate on the frame 80. For example,the solar collectors 55 are disposed in parallel to one another betweenthe second opposing ends 84 of the frame 80.

The solar collectors 55 can have a number of features. For example, somesuitable features for the solar collectors 55 can be found in US2010/0108124, US 2012/0024374, and US 2012/0312351, each of which isincorporated herein by reference in its entirety. As discussed in moredetail later with reference to FIGS. 3A-3C, each solar collector 55 canbe a solar concentrator having reflective surfaces and photovoltaiccells. Additionally, each collector 55 can have either an axle or axlemount (not shown) at both ends wherein the axles or axle mounts forms asecond axis A₂ of rotation along the longitudinal axis of collector 55.This second axis A₂ of rotation is carried by the first axis A₁ ofrotation.

In one embodiment as shown, frame 80 is rectangular in shape, althoughother shapes can be used. A side 82 of frame 80 can be aligned to truenorth with the plane of the frame 80 rotatable east and west relative tothe ground, a building rooftop, or other surface (not shown). In oneembodiment, a side 82 of frame 80 is aligned to be parallel to the axisof rotation of the earth, and the solar collectors 55 can beperpendicular to the side 82 of frame 80. In one embodiment, the firstaxis A₁ of frame 80 is aligned with true north and is placed with thenorth end of the frame 80 elevated higher relative to the ground thanthe south end of the frame 80 (in the Northern hemisphere). In oneembodiment, daily rotational points 75, 85 are aligned parallel to therotational axis of the Earth.

Hoop pulley 60 is connected to the first drive having daily drive motor65 by wire cable (not shown) affixed to hoop pulley 60. Daily drivemotor 65 is operative to rotate and thereby move hoop pulley 60 toorient the frame 80.

In the case where daily rotational points 75, 85 are aligned parallel tothe rotational axis of the Earth, for example, frame 80 must rotate onits axis A₁ every day to keep the frame 80 aligned to the sun as the daypasses. In this instance, solar collectors 55 will move only slightly intilt during a single day. Solar collectors 55 must change tilt angleabout their axes A₂ as the seasons progress because the apparentposition of the sun in the sky changes. Because of thesecharacteristics, the first axis A₁ is referred to as the daily axis, andthe second axis A₂ of solar collectors 55 is referred to as the seasonalaxis, even in those cases in the present application where the axis A₁of the frame 80 is not parallel to the axis of rotation of the Earth.

In one embodiment, the actuator 65 can be a daily drive motor, such astepper motor that divides a full rotation into a number of equal steps.The stepper motor's position can then be commanded to move and hold atone of these steps without any feedback sensor (open loop) as long asthe stepper motor 65 is carefully selected. As will be appreciated,other actuators and motors can be used.

FIG. 2A shows one system 50 on its own. A typical installation at a sitewill have a plurality of these systems 50 disposed adjacent one anotherin an array or a matrix to provide increased surface area for thecollection of solar energy. Each of the various systems 50 can bemounted end-to-end and adjacent one another in as much as a packedmanner as possible as long as consideration is given to possibleovershadowing of the systems 50 with one another.

Each of the systems 50 for such an installation can be self-contained asshown in FIG. 2A. However, several of the systems 50 can be integratedtogether so as to connect and/or operate together, which can simplifyinstallation, maintenance, and operation. For example, FIG. 2B depictstwo solar concentrator systems 50A-B of FIG. 2A integrated with oneanother. (Although only two systems 50A-B are connected in FIG. 2A,other installations may interconnect more than two systems 50 dependingon the size, weight, and other factors involved.)

One of the systems 50A has the first drive with the daily drive motor65, and the other system 50B relies on that daily drive motor 65. Inthis way, the one motor 65 can drive both frames 80 a and 80 b,resulting in an inherent cost reduction. The solar systems 50A-B asbefore include stands 70 a and 70 b, daily rotational points 75, andsolar collectors 55. The systems 50A-B share the one daily drive motor65, the hoop pulley 60, and a central upright 76. Stands 70 a and 70 bare connected so as to form a single supporting unit for solarconcentrator system 51. Central upright 76 is affixed to stand 70 a byclamps or welding. Hoop pulley 60 is affixed to frame 80 a and is drivenby daily drive motor 65. Frame 80 a is rigidly connected by connectors,slates, plates, etc. (not shown) so that rotation of one frame 80 aabout daily rotational points 75, 85 induces the same motion of theother frame 80 b. For example, the connector may comprise a plateattached to both frame 80 a and frame 80 b. Frames 80 a and 80 b eachrotate about a central daily axle (not shown) affixed to central upright76. Other considerations noted above with respect to the solarconcentrator system 50 of FIG. 2A apply to each of these interconnectedsystems 50A-5B.

As noted above, each of the solar collectors 55 for the system 50 can bea solar concentrator. FIG. 3A depicts an overview of a solarconcentrator 100 according to the present disclosure. The solarconcentrator 100 includes at least one reflector 110 disposed on a firstface 122 of the solar collector 100 and includes at least onephotovoltaic cell 105 disposed on a second face 124 of the solarcollector 100 angled adjacent the first face 124. The at least onephotovoltaic cell or photovoltaic unit 105 is disposed at a focus of theat least one reflector 110 and converts solar energy reflected theretointo photovoltaic energy.

As shown in particular in FIG. 3B, solar concentrator 100 has a tray orhousing unit 125 with opposing ends 120 and with the adjacent first andsecond faces 124, 125 extending between the opposing ends 120. Theadjacent first and second faces 124, 125 defines an open trough 123 ofthe tray or housing unit 125. The opposing ends 120 having thirdrotational points rotatably connect to the opposing sides of the frame(80), as noted in more detail later.

As shown, the at least one reflector 110 comprises a plurality ofconcave reflective surfaces or mirrors 110 disposed along a length ofthe housing unit 125. The at least one photovoltaic cell or photovoltaicunit 105 thereby includes a plurality of the photovoltaic cells eachdisposed relative to one of the concave reflective surfaces 110. Each ofthe concave reflective surfaces 110 comprises gain in two optic axesconcentrating solar energy to the relative photovoltaic cell 105.

As shown in particular in FIGS. 3A-3C, the housing unit 125 having theconcave reflective surfaces 115 on the reflector 110 and having thephotovoltaic unit 105. The housing unit 125 has end caps 120 at theopposing ends and has axle 150 as rotational points. Photovoltaic unit105 is disposed so as to receive concentrated solar radiation (notshown) reflected by reflective surfaces 115 and to convert same intoelectrical energy. Reflective surfaces 115 may be spherical oraspherical mirrors. In one embodiment, reflective surfaces 115 arerotationally symmetrical.

FIG. 3B depicts an exploded diagram of solar concentrator 100 havingcover glass 130, housing 125, end caps 120, a plurality of reflectivesurfaces 115, and photovoltaic unit 105. Cover glass 130 acts as aweather shield for the components of solar converter unit 100.

FIG. 3C depicts a cross-sectional view of the optical functions of solarconcentrator 100. Solar rays a and b pass through cover glass 130 andare reflected by reflector surfaces 115 to photovoltaic unit 105 affixedto housing assembly 125 where the solar rays a and b are converted toelectrical energy. Arrow c represents the optic axis of reflectivesurface 115.

The reflective surfaces 115 can have a number of shapes, and each of thereflective surfaces 115 can have the same or different shape from oneanother depending on the advantages. In one embodiment, for example,each of the reflective surfaces 115 is a parabolic mirror with gain intwo axes. Alternatively, reflective surfaces 115 can each be a parabolicmirror with rotational symmetry or a parabolic mirror with rotationalsymmetry where at least one aspheric constant is not equal to zero.

In one embodiment, the surface formula for the reflective surface 115 isgiven by:

y(x)=x ²÷(R(1+√(1−(1+K)x ² ÷R ²)))+α₁ r ²+α₂ r ⁴

-   -   K (conic constant)=−0.986    -   R (radius)=75.7    -   α₁ (aspheric constant)=1.084×10⁻⁴    -   α₂ (aspheric constant)=1.129×10⁻¹⁰    -   Focal length=37.30

The advantage of the use of aspheric constants in the design ofreflective surface 115 is that they can be used to make the spot oflight more uniform, which increases the harvesting efficiency of thesolar concentrator units and prevents damage resulting from highintensity spikes within the spot of light.

FIG. 4A depicts a first end of solar concentrator 100 (not shown)comprising housing 125, reflective surfaces 115, end cap 120, clamp 135,flange 140, screws 145 and pressure relief cap 131. Clamp 135, flange140 and screws 145 are used for attaching the depicted end of solarconcentrator 100 (not shown) to pulley axle 183 (not shown). Pulley axle183 (not shown) is inserted between clamp 135 and flange 140, and screws145 are then tightened by a screw driver or similar instrument (notshown) to form a tight bond around pulley axle 183.

FIG. 4B depicts a second end of solar concentrator 100 (not shown)comprising housing unit 125, reflective surfaces 115, end cap 120,hollow axle 150, and wiring 155 routed through hollow axle 150. Axle 150is inserted into bearing mount 184 (not shown). All electricalconnections to photovoltaic cells (not shown) that form a part of solarconcentrator modulation 100 are made through wiring 155. In oneembodiment, wiring 155 is a wiring material capable of withstandrepeated flexing.

FIG. 4C depicts a pulley 180/pulley axle 183 combination mounted ontobearing mount assembly 182. Bearing mount assembly 182 is affixed toframe (80) by mounting hardware (not shown), such as a bolt, nut, andwasher combination inserted through mounting hole 181.

FIG. 4D depicts bearing mount assembly 184. Hollow axle 150 mounts intobearing mount assembly 184 through aperture 186. In one embodiment, thebearing mount assemblies 182 and 184 are fabricated to identicalspecifications. Bearing assembly 184 may be fabricated by inserting acommonplace ball bearing with inner and outer races into a housing.Fixing of the bearing in the housing may be as simple as first loweringthe temperature of the bearing in a refrigerator before inserting thebearing into the cavity in the housing. When the bearing returns tonormal temperature, it will fit snuggly into the cavity. In oneembodiment, the bearings on the pulley side are metal and the bearingson the axle end are plastic. In one embodiment, the pulley bearings onthe solar concentrator module closest to the driver pulley are metal andthe pulley bearings on the end opposite the driver pulley are metalwhereas all other pulley bearings are plastic. Bearing mount assembly184 may be affixed to frame (80) by mounting hardware (not shown), suchas a bolt, nut, and washer combination inserted through mounting hole185.

FIG. 5A depicts the solar concentrator system 50 again showing base unitor stand 70 with drive motor 65 and rotational points 75 affixedthereto, and showing frame 80 with concentrators 55 and hoop pulley 60affixed thereto, the frame 80 being mounted on rotational points 75. Asnoted, each concentrator 55 comprises either an axle or axle mount (notshown) at both ends wherein the axles or axle mounts forms a point ofrotation along the longitudinal axis of concentrator module 55. In oneembodiment, the long side of frame 80 is aligned with true north withthe plane of the frame parallel to the ground or a building rooftop (notshown). In one embodiment, the long side of frame 80 is aligned to beparallel to the axis of rotation of the earth. In one embodiment in theNorthern hemisphere, the long axis of frame 80 is aligned with truenorth and is placed with the north end of the frame elevated higherrelative to the ground than the south end of the frame. Hoop pulley 60is connected to drive motor 65 by wire cable 90 affixed to hoop pulley60 (not shown). Drive motor 65 is operative to rotate and thereby movehoop pulley 60.

Drive motor 65 can be used to drive rotational points 75 directly.Although this may be possible, it is preferred that the pulleyarrangement disclosed herein is used to provide the mechanical advantageoffered by the pulley arrangement herein presented.

It is envisioned that the optic axis (c of FIG. 3C) of each reflectivesurface (115 of FIG. 3C) of each solar concentrator (124 of FIG. 3C) onevery solar concentrator module 55 is substantially parallel to theoptic axis (c of FIG. 3C) of each concentrator reflective surface (115of FIG. 3C) of every other solar concentrator (124 of FIG. 3C) of everyother solar concentrator module 55 of the system 50. In a properlydesigned system according to the present disclosure, this condition willbe met whether operation is dynamic (rotating) or static.

FIGS. 5B and 5C depicts end views of solar concentrator system 50showing the frame 80, rotational points 75, base unit 70, drive motor 65and hoop pulley 60. Hoop pulley 60 is affixed to frame 80 such thatdrive motor rotates frame 80 on rotational points 75 by one or more wirecables (not shown). Drive pulley 67 is affixed to driver motor 65.Together frame 80, rotational points 75, 85, drive motor 65, and hooppulley 60 constitute a daily drive system operative to rotate frame 80about rotational points 75, 85 in response to commands from a controlsystem (not shown).

FIG. 5D depicts an arrangement of the rotations points 75, 85 betweenthe stand 70 and frame 80. The first rotational point 75 of the stand 70includes slots 575 defined in a central post 71 of the stand 70. Thesecond rotational point 85 of the frame 80 includes a bearing assembly587 disposed on the frame's end 84 and includes an axel 585 extendingfrom the bearing assembly 587. For assembly, the axle 585 fits into theopen slot 575, and an end cap 573 can affix on the post 71. Each side ofthe post 71 may have slots 575 so the system can readily support theaxels 585 of two adjoining frames 80 in the combined configuration notedin FIG. 2B.

FIG. 5E depicts one way the one or more wire cables 90A-B can connect toa drive pulley 67 for the hoop pulley 60, such as in FIG. 5C. Here, thedrive pulley 67 includes a pulley body 567 having two segments of cable90A-B wrapped in opposing directions about the outer circumference. Onecable 90A affixes with a fixture 92 at one end of the body 567 and wrapscounter clockwise around the circumference from the outer edge towardthe center. Another cable 90B affixes with a fixture 92 at the oppositeend of the body 567 and wraps clockwise around the circumference fromthe outer edge toward the center. As the pulley body 567 rotatescounter-clockwise, the one segment 90A pays out, while the other segment90B winds up. As the pulley body 567 rotates clockwise, the one segment90A winds up, while the other segment 90B pays out. As an alternative tothe two cable segments 90A-B, one continuous cable can be used withportion of the cable wrapped around the drive pulley 567. A portion ofthe continuous cable can be affixed to the pulley 567 in a number ofways.

The drive pulley 67, the wire cable 90, and the hoop pulley 60 can havea number of configurations and can use a number of tensioningarrangements. For example, FIGS. 6A-1 and 6A-2 depict one arrangement ofwire cable 90 on hoop pulley 60. Wire cable 90 is affixed to the twoends of hoop pulley 60 by a fastener, such as a clamp or a screw (notshown). Wire cable 90 is deployed along the circumference of hoop pulley60 and is wrapped around driver pulley 67. In one embodiment, driverpulley 67 is a helical grooved pulley. Tensioning spring 86 keeps wirecable 90 taut by applying tension on driver pulley 67. Driver pulley 67is affixed to drive motor 65 (not shown). Wire cable 90 is affixed toboth ends of hoop pulley 60 by a fastener, such as a screw or clamp (notshown). Hoop pulley 60 has substantial breadth. One end of wire cable 90is affixed to a first edge of hoop pulley 60 that is closes to theinterior of frame 80 and the other end is affixed to the edge of hooppulley 60 that is opposite the first edge of hoop pulley 60. Drivepulley 67 is configured with helical grooves deployed such that as wirecable 90 is driven by driver pulley 67 the position of wire cable 90 ondrive pulley 67 remains in a line with the position of wire cable 90 onhoop pulley 60.

FIG. 6B depicts an alternate way for tensioning wire cable 90. Wirecable 90 is affixed to hoop pulley 60 by a fastener, as previouslystated. Wire cable 90 is routed to driver pulley 67 which is affixed todrive motor 65 (not shown). Wire cable 90 is wrapped about drive pulley67 at least once in the direction of each end of wire cable 90 and thenis further routed to tensioner pulley 87. In one embodiment, wire cable90 is only wrapped about tensioner pulley 87 by a half turn. Tensionerspring 86 hold wire cable 90 taut by pulling on tensioner pulley 87.

FIG. 6C depicts yet another alternate way for tensioning wire cable 90.Wire cable 90, driver pulley 67, and hoop pulley are deployed as inprevious examples. Wire able 90 is affixed to one end of tensioningspring 95 and the other end of tensioning spring 95 is affixed to hooppulley 60. Optionally a second tensioning spring 97 may be deployed byaffixing the remaining end of wire cable 90 to one end of tensioningspring 97 and the other end of tensioning spring 97 is affixed to theremaining end of hoop pulley 60.

FIG. 6D depicts another alternate way for tensioning wire cable 90. Wirecable 90 is double wrapped on drive pulley 67. Wire cable 90 is deployedalong hoop pulley 60 and then extended beyond hoop pulley to tensioningspring 88. A first end of wire cable 90 is affixed to a first end oftensioning spring 88 and the remaining end of wire cable op is affixedto the remaining end of tensioning spring 88. In one embodiment, wirecable 90 is affixed at one end of hoop pulley 60 by a fastener; such asa screw or clamp (not shown).

FIGS. 6E and 6F depict additional details of the arrangement of wirecable 90, hoop pulley 60, and drive pulley 67. In particular, FIG. 6Efurther illustrates the point of attachment for wire cable 90 to hooppulley 60. Flange 89 is provided to insure that wire cable 90 staysrouted around the circumference of hoop pulley in the even wire cable 90is momentarily slack. For its part, FIG. 6F depicts the helical groovestructure of drive pulley 67, as well as one way of wrapping acontinuous wire cable 90 thereon, as opposed to the arrangement of FIG.5E.

As alluded to above with reference to FIG. 2A, the module 80 of thesystem 50 is a frame rotatably mounted on the stand 70. FIGS. 7A and 7Bdepict frame 80 and rotational points 85. Again, the frame 80 has secondopposing ends 84 and has opposing sides 82. Several of the solarcollectors 55 are not shown so the structure of the frame 80 can be moreclearly seen. However, as noted, the solar collectors 55 are rotatablealong second parallel axes between the opposing sides 82.

A plurality of pulleys 180 are disposed along at least one of theopposing sides 82 and are connected to the second axes A₂ of the solarcollectors (55). A second drive 104 disposed on the frame 80 is operableto provide second rotation, and at least one second cable (not shown)connected between the second drive 104 and the pulleys 180 rotates thesolar collectors (55) in tandem about the second axes A₂ in response tothe second rotation. [00129] in particular as shown, a series of drivenpulleys 180 mounted along the side of frame 80 are depicted. Seasonaldrive motor 104 is affixed to frame 80 in at a position where a driverpulley (not shown) may be positioned in line with driven pulleys 180.

FIG. 7C depicts frame 80 and hoop pulley 60 and the location of drivermotor 104. Driver motor 104 is connected to a driver pulley (not shown)which serves to drive the series of driven pulleys (180) which in turnrotate concentrators (55). Driver motor 104 is referred to as theseasonal driver motor because it drives the concentrator modules (55) ontheir seasonal axes A₂.

In one embodiment, driver motor 104 is a stepper motor that enables themotor to be driven to a known position. A stepper motor divides a fullrotation into a number of equally spaced steps. A commonly used numberof steps is 200, so that each full step corresponds to 1.8°. A steppermotor may then be driven to a precisely known shaft position based onthe number of pulses it is driven by. Such motors are well known in theart. A properly designed stepper motor control system comprisingcontroller hardware and firmware may drive the stepper motor from afirst known position corresponding to a first angle to a second knownposition corresponding to a second angle. This enables coordinationbased on angles in degrees or in other units of angular measurement suchas radians, mils, grad, or arbitrary units. As an alternative to using astepper motor alone, the drive may use an angular positioning systembased on a motor, a circular or shaft encoder, and a motor controlsystem having hardware and firmware operative to read the circularencoder and drive the motor from a first position corresponding to afirst angle to a second known position corresponding to a second angle.A motor for such implementations may be a brushless DC motor, abrushless AC motor, a brushed DC motor, or other motors known in theart. An angular position drive system for such an arrangement wouldinclude a motor, a motor controller and control hardware and firmwareoperative to move the optic axis of the solar concentrator (55) from afirst angle to a second angle. In the present disclosure, all referencesto firmware are understood to include both firmware and software as bothare machine executable code.

As will be appreciated, separate drives, such as a single stepper motor,can drive each individual concentrator module (55) directly. Thisrequires coordinated control, increased complexity, more weight andhardware, etc. For this reason, driving a large number of concentratormodules (55) with one drive is preferred using the cable and pulleyarrangement disclosed herein.

FIG. 7D depicts a section of frame 80 located adjacent to hoop pulley60. Driver pulley 185 is connected to driver motor (104). Driven pulleys180 are driven by driver pulley 185 through a wire cable 91 thatconnects them together. Fastener 92 in this case is a screw operative toclamp wire cable 91 to driven pulley 180.

FIG. 7E depicts driven pulleys 180 and 180 a connected by wire cable 91.Wire cable 91 is wrapped once around all driven pulleys 180 and wrapped1I times around driven pulley 180 a which is the last of the drivenpulleys. Drive pulley 180 a is the last driven pulley at the oppositeend to driver pulley (185). Fastener 92 is used on driven pulley 180 tosecure the position of wire cable 91. A fastener is not used on drivenpulley 190 a because it has been demonstrated not to afford a sufficientrotation. Increasing the number of turns of wire cable 91 about drivenpulley 180 a to 2½ turns would overcome this limitation and allow theuse of fastener 92 on driven pulley 180 a. Driver pulley 180 and drivenpulley 180 a are identical in all manufacturing respects.

FIG. 7F depicts a single driven pulley 180 wherein wire cable 91 isanchored to driven pulley 180 by fastener 92. Wire cable 91 (representedby a dashed line) is wrapped once around driven pulley 180. Aconcentrator module 55 (not shown) is attached to each driven pulley180. Each driven pulley 180 with concentrator module (55) affixedthereto may rotate slightly less than 180° in each direction from thepoint where fastener 92 is directly opposite the point where wire cable91 first touches driven pulley 180 in each direction.

FIG. 7G depicts an alternate way of moving a wire cable 91 according tothe present disclosure. Linear actuator 350 comprises a linear steppermotor attached at two points to cable 91. Driven pulleys 181 a-e areoperative to rotate solar concentrators (55). Idler pulley 351 serves topull the return line for cable 91 away from driven pulleys 181 a-e.Linear actuator 350 is ideally affixed to the same frame that all thepulleys 181 a-e are attached to, although the disclosed arrangement willoperate correctly provided the position of linear actuator 350 relativeto all of the pulleys regardless of the point of attachment. In oneembodiment, idler pulley 351 serves as part of a tensioning systemdesigned to keep cable 91 taut.

FIG. 7H presents more detail of linear actuator 350. Linear actuator 350comprises motor 352, shaft 354, and anti-rotational sleeve 353 attachedto motor 352. Anti-rotational sleeve 353 is operative to prevent shaft354 from rotating while moving. This is advantageous because twistingcable 91 of FIG. 7F may have an adverse impact on cable lifetime as wellas on the smooth function of the moving of the cable. There are severalconfigurations of linear actuator 350 that can achieve this. In oneexample, for example, motor 352 may rotate a worm gear that operates ona series of grooves in shaft 354.

FIG. 7I depicts a cross-section of one configuration to prevent rotationof a shaft. Shaft 355 passes through anti-rotational sleeve 356.Rotation is controlled by key 357, which passes through anti-rotationalsleeve 356 and seats into a groove of rectangular cross section in shaft355.

Various tensioner arrangements can be used to tension the wire cable 91for the seasonal drive system. For example, FIG. 8A depicts a fullseasonal drive system according to the present disclosure comprisingdriven pulleys 181 a-e, wire cable 91, driver pulley 185, and tensioningspring 165. Placing tensioning spring 165 at this position may result inunequal force depending on which direction driver pulley 185 isrotating. Therefore, alternate ways of tensioning can be used to reducethe degree of unequal force in the two directions of rotation.

In another example, FIG. 8B depicts a full seasonal drive systemcomprising driven pulleys 181 a-e, wire cable 91, driver pulley 185eyelet bolts 171 having reverse threads from each other and turnbuckle164. Placing turnbuckle 164 at this position in place of tensioningspring 165 of FIG. 8A can eliminate the issue of unequal force dependingon which direction driver pulley 185 is rotating.

In other examples, FIGS. 8C-1, 8C-2, and 8C-3 depict three views of afull seasonal drive system for a solar concentrator system, such assystem 50 of FIG. 2. View 160, view 161, and view 162 of the seasonaldrive system each comprise driven pulleys 181 a-e, driver pulley 185,tensioner pulley 190, and wire cable 91. View 160 further comprisestensioner (compression) spring 166. Driven pulley 160 a receives a fullwrap and an additional half wrap of wire cable 91. The additional halfwrap is helpful because wire cable 91 changes direction at driven pulley181 a. Driven pulleys 181 b-e each receive one full wrap as can be seenin view 161. Both upper and lower parts of wire cable 91 are wrapped onfull turn around driver pulley 185 and then connect by one half turnaround tensioner pulley 190. View 161 presents additional detail aboutwire cable 91 and its wrap around driver pulley 185. Tensioner spring166 exerts force on tensioner pulley 190 to assist in keeping wire cable91 taut. View 161 depicts from above the layout of wire cable 91 underdrive pulleys 181 a-e, driver pulley 185, and tensioner pulley 190. Inone embodiment, tensioner spring 166 may be replaced by a turnbucklesuch as turnbuckle 164 of FIG. 5B.

FIGS. 8D and 8E depict two alternative methods of deploying tensionerpulley 190 in the example of FIGS. 8C-1, 8C-2, and 8C-3. In FIG. 8Dtensioner pulley 190 is located on the opposite side of driver pulley185 from driven pulleys 181 a-e. Tensioner spring 165 is placed in lineon wire cable 91 between driver pulley 185 and in line tensioner pulley190. Wire cable 91 is wrapped about driven pulleys 181 a-e and driverpulley 185 as disclosed in FIGS. 8C-1, 8C-2, and 8C-3.

FIG. 8E depicts a substantial alteration from FIGS. 8C-1, 8C-2, and8C-3. Tensioner pulley 190 is placed at the opposite end from driverpulley 185. Driver pulley 185 and driven pulleys 181 b-e aresubstantially the same as previously disclosed. Driven pulley 181 a isdouble wrapped as previously described for driver pulley 185 in FIGS.8C-1, 8C-2, and 8C-3. Tensioner pulley 190 is wrapped as previouslydescribed in FIGS. 8C-1, 8C-2, and 8C-3. Tensioner spring 166 acts upontensioner pulley 190 to keep wire cable 91 taut.

FIG. 5F depicts anther method of tensioning wire cable 91. The exampleof FIG. 8F replaces tensioner pulley 190 and tensioner spring 166 withan alternative arrangement comprising idler pulley 169 and tensionerpulley 168. Tensioner pulley 168 is acted on by compression spring 167to depress any slack in wire cable 91 and thereby keep wire cable 91taut.

FIG. 8G depicts details regarding the wire cable 91 double wrap ofdriver pulley 185. Driver pulley 185 comprises a spiral groove formedonto a cylindrical surface. In the example of driver pulley 185 thespiral groove comprises six complete turns. For purposes of thisdisclosure, the grooves are designated by letters a through f beginningat the side of pulley 185 away from the frame (80). Each letterdesignated segment comprises one full revolution of the spiral beginningat the bottom of driver pulley 185 as depicted in FIG. 8G. Spiralsegments a and b are not used. Wire cable 91 is routed from tensionerpulley 190 (not shown) and comes into contact with driver pulley 185 atsegment d. Wire cable 91 wraps around one turn of a groove of driverpulley 185 and breaks contact with driver pulley 185 at segment e,whereupon it is routed to driver pulleys, such as 181 e (not shown). Thereturn routing of wire cable 91 first makes contact with driver pulley185 at a position near the boundary of segment b and segment c. It wrapsaround driver pulley 185 in segment c and breaks contact underneathdriver pulley 185 near the boundary between segment c and segment d. Itthen is routed to tensioner pulley 190 (not shown). Thus, wire cable isdouble wrapped around driver pulley 185, once in each direction.

One issue with cable drive systems is that wire cables may stretch overtime. The forces acting on the cable include stretching due to tension,torque and the force of gravity on the wire. While not presenting ashort term issue, it is useful to find solutions that can be appliedover the lifetime of the solar concentrator system 50 without requiringmajor maintenance. This problem may be addressed through a combinationof components. Provision for tightening with a tensioner is one aspectof a solution. A second aspect is to reduce the length of wire cablewhere possible.

As one example, FIG. 81I presents the driver cable routing for a solarconcentrator system similar to the driver system depicted in FIG. 8BFIG. 8B depicts a full seasonal drive system comprising driven pulleys181 a-e, wire cable 91, driver pulley 185, and a tensioner assemblycomprising assembly rod 163, assembly rod 170, and turnbuckle 159.Assembly rod 163 is configured with right hand threads and assembly rod170 is configured with left hand threads so that the tension on wirecable 91 may be changed by rotating turnbuckle 159 without causingrotation of either assembly rod 163 or assembly rod 170. Use of longerrods reduces the length of wire cable 91 by the length of the two rods.

Use of the longer tensioner system requires attaching wire cable 91 toeyelets formed in the ends of assembly rods 163. FIG. 8I demonstrateshow a loop may be formed by placing ferrule 191 onto wire cable 189,passing wire cable 189 through the eyelet of one of assembly rods 163,and then passing wire cable 189. Afterwards ferrule 191 may be crimpedor otherwise secured to wire cable 189. Assembly rods 163 and turnbuckle159 are depicted in FIG. 8J.

Even with the use of a rod the force of gravity on the overall length ofthe return line of wire cable 91 remains. One way to mitigate the effectof gravity is to route each of assembly rods 163 through a suitablyplaced eyelet bolt 192 as depicted in FIG. 8K. Positioning the eyeletbolts such that each of assembly rods 163 is support along its normalreturn path to driver cable 185.

The subject of periodic maintenance is of great importance in theoperation of the solar concentrator system 50. As discussed, the system50 is often positioned in relatively inaccessible locations such as aroof top with no convenient way of access. During installation of thesystem 50, it is useful if a way to adjust tension on the cable isprovided. Wire cables are known to stretch slightly over time even ifpre-stretched to minimize the effect.

For this purpose, FIG. 8L depicts tension assembly or apparatus 330operative to maintain tension on a wire cable such as wire cable 91 ofFIG. 8H in an autonomous manner. Assembly 330 may replace turnbuckle 159and may also replace assembly rods 163 when suitably configured (notshown). Assembly 330 may be used in any application in which a wirecable or any other type of cable must be maintained in a taut state overa period of time and is not limited to solar energy applications.Tension assembly 330 acts to maintain tension in a cable by working toshorten the distance between ends 331 and ends 332 in response to anincrease in slack in the wire cable (not shown). In this way, thetension assembly 330 is operative to maintain tension on a cable.

The assembly 330 includes a housing 338, a piston 342, a first biasingelement 334, a catch 339, and a second biasing element 335. The housing338 has a first end connected to a first portion of the cable (91). Thepiston 342 is at least partially movable in the housing 338. The piston342 has a second end connected to a second portion of the cable (91).The first biasing element 334 is disposed in the housing 338 and biasesthe piston 342 toward the first end of the housing 338.

The catch 339 is disposed in the housing 338. The catch 339 isengageable against the piston 342 moved in a first direction and isdisengagable from the piston 342 moved in a second direction opposite tothe first direction. The second biasing element 335 disposed in thehousing 338 biases the catch 339 to engage the piston 342.

As depicted in particular in FIG. 8L, tension assembly 330 comprisespiston 342, housing 338, biasing elements or compression springs 334 and335, and catch or brake components 341 a and 341 b. Piston 342 comprisespiston shaft 333, piston platen 336, and piston eyelet 332. Housing 343comprises housing cylinder 338, end cap 344, eyelet 331, internal platen337, and end platen 340. Internal platen 337, end cap 344, and endplaten 340 are rigidly fixed to housing cylinder 338 by welding,adhesion, and the like. Those of skill in the art having the benefit ofthe present disclosure will recognize other ways of fabricating thesimilar structures. Eyelet 331 is affixed to end cap 344. Internalplaten 337 and end platen 340 are fabricated with apertures. Theaperture (not shown) of internal platen 337 is fabricated such thatpiston shaft 333 of piston assembly 342 passes through the aperture.

Compression coil spring 335 is positioned around piston shaft 333 andbetween piston platen 336 and interior platen 337. Because interiorplaten 337 is fixed, coil spring 335 is operative to press upon pistonplaten 336 and potentially move piston assembly 342. Compression coilspring 334 is positioned around or adjacent to piston shaft 333 andbetween interior platen 337 and wedge brake or slips 341 a. Wedge brakeor slips 341 a comprises at least one cone/cylinder unit which may besegmented parallel to the axis of piston shaft 333 such that it may,when under pressure from coil spring 334, seat firmly in brake housing341 b. In another embodiment, the wedge brake 341 a may comprise two ormore components, slips, wedges, or the like. The term wedge brake asused for this component is selected for the purposes of discussion andis not intended to imply that a wedge brake has all of the attributes ofa wedge as is normally understood in the field of mechanics. When wedgebrake 341 a is pressed into brake housing 341 b, it applies a level ofpressure to piston shaft 333 which impedes movement of piston shaft 333.

The shape of the cavity of brake housing 341 b is substantially the sameas that of the exterior surface of wedge brake 341 a. The shape of theinterior of wedge brake 341 a is substantially that of the exterior ofpiston shaft 333 in that piston shaft 333 should pass through wedgebrake 341 a when wedge brake 341 a is under a lower level of pressure.End platen 340 serves to keep brake housing 341 b in place. End platen340 is fabricated with an aperture such that the cylindrical portion ofwedge brake 341 a passes through it while being held substantially inplace.

Eyelet 332 and eyelet 331 are each attached to a wire cable (not shown)through the use of a loop such as that of FIGS. 8H and 8I. As will beappreciated with the benefit of the present disclosure, there arealternatives other than the eyelet loop for fastening a cable or springto another element of a system. It is intended that a reference to aneyelet encompasses alternative fasteners.

An understanding of the functioning of tension assembly 330 requiresknowledge of the forces applied to the assembly. External forces (markeda and b) are applied by the aforesaid wire cables (not shown) attachedto the assembly at eyelets 331 and 332. In all but transitory phenomena,such as movement of the wire cable, forces a and b are equal to eachother, thus keeping tension assembly under fixed external force. Themagnitude of the external force on tension assembly 330 equals a+b.

Internal force is applied to tension assembly 330 by coil compressionspring 335 which acts to maintain or reduce the distance between eyelet331 and 332. Coil compression spring 335 places pressure d on pistonplaten 336. Pressure d exerted by coil compression spring 335 acts tomove piston assembly 342 further into tensioner system 330 which wouldreduce the distance between eyelet 331 and eyelet 332 thereby increasingthe tension on the wire cable (not shown) which means increasing theforce between a and b.

Coil compression spring 334 places pressure with force c on wedge brake341 a. Brake housing 341 b directs wedge brake 341 a into firmer contactwith piston shaft 333 which in turn stops movement of piston assembly342 in an outward direction. The actual force exerted by wedge brake 341a on piston shaft 333 will continually increase as increasing outwardpull is exerted on eyelet 332 by the wire cable (not shown). This showsthat the action of wedge brake 341 a on piston shaft 333 is sufficientto prevent piston assembly 342 from moving further out of tensionassembly 330. Therefore, lengthening of the distance between eyelet 331and eyelet 332 is prevented.

The force exerted by wedge brake 341 a on piston shaft 333 when pistonshaft 333 is caused to move further into tension assembly 330 is limitedto that required to overcome a level of friction between piston shaft333 and wedge brake 341 a and between brake wedge 341 a and brakehousing 341 b.

Once this friction is overcome, piston assembly 342 will move as farinto tension assembly 330 as other forces discussed hereafter permit.The nature of the friction that must be overcome depends on a number ofdifferent factors, which include the nature and the finishes of thematerials of which piston shaft 333, wedge brake 341 a and brake housing341 b are fabricated and which also include the effective apex angle ofthe conical portions of wedge brake 341 a and brake housing 341 b. Thisshows conclusively that piston assembly 344 will retract further intotension assembly 330 if the force applied to piston eyelet 332 by thewire cable (not shown) is not sufficient to prevent force d applied bycoil compression spring 335 from overcoming the friction between pistonshaft 332 and wedge brake 341 a and between wedge brake 341 a and brakehousing 341 b.

A consideration is to accommodate a need to reset the position of pistonassembly 342 relative to cylinder 343. One way for doing this is topress the parts of wedge brake 341 a that extends beyond exteriorhousing platen 340 toward the interior of tension assembly 330. Thisallows the position of piston assembly 342 to be moved either in or outas needed. Once the reposition is finished pressure on wedge brake 341 acan be removed after insuring that the wire cable (not shown) isattached.

Thus, tension assembly 330 has been shown to maintain its length whenthe wire cable (not shown) is sufficiently taut and to be able toshorten its length autonomously when the wire cable is overly slack,thereby increasing the tension on the wire cable.

Those of ordinary skill in the art will recognize with the benefit ofthe present disclosure that an extension spring placed so as to pullpiston assembly 342 toward piston end cap 344 would cause tensionassembly 330 to function in a similar manner. In one embodiment, pistonplaten 336 is eliminated and an eyelet is added. Those of skill in theart will recognize that in one embodiment both extension and compressionsprings may be used. There may be reasons to prefer one approach overthe other, but all can be made to work.

FIG. 8M depicts an alternative tension assembly 348 according to thepresent disclosure. Tension assembly 348 comprises piston 349, housing338, first biasing element or compression spring 334, second biasingelement or extension spring 347, and catch or brake components 341 a and341 b. Piston 349 comprises shaft 358, piston eyelet 345, and pistoneyelet 332. Housing 343 comprises housing cylinder 338, end cap 344,eyelet 331, internal platen 337, and end platen 340. Internal platen337, end cap 344, and end platen 340 are rigidly fixed to housingcylinder 338 by welding, adhesion, and the like. Other ways offabricating the similar structures can be used. Eyelets 331 and 346 areaffixed to end cap 344 externally and internally respectively. Internalplaten 337 and end platen 340 are fabricated with apertures. Theaperture (not shown) of internal platen 337 is fabricated such thatpiston shaft 333 of piston assembly 349 passes through the aperture.

The function of commonly numbered elements of FIG. 8L and FIG. 8M arethe same. The only substantial point of difference is that internalforce d is generated by the action of extension spring 347 to contract.Internal force c is identical between the embodiment of FIG. 8L and FIG.8M as are external forces a and b. The functionality of the twoembodiments is identical in concept.

Platen 336 of piston assembly 343 and eyelet 345 of piston assembly 349perform the same function, which is to enable compression spring 335 andextension spring 347 to exert force upon their respective pistonassemblies tending to move the piston assemblies 343 and 349 deeper intotheir respective housing assemblies 343 and 343.

Returning to a discussion of the disclosed solar collector 100, FIGS. 9Aand 9B depict perspective and side views of solar collector 100comprised of mirror assembly 115, module housing 125, end cap 120, andbrush assembly 126. FIG. 9B further depicts bottom flange 140 affixed toend cap 120 (not shown) with clamp 135 affixed to bottom flange 140 byfastener 145 (not shown). FIG. 9B additionally depicts axle 150 andwiring 155 at the opposite end of solar collector 100.

FIG. 9C depicts end cap 120 with bottom flange 140 affixed thereto.Clamp 135 is affixed to bottom flange 140 by fastener (screws) 145.Pressure relief cap 131 is affixed to end cap 120. Brush mountingfeature 127 forms part of module housing 125 (not shown) and brushassembly 126 is inserted into brush mounting feature 127 which holds itsecurely.

One ongoing problem with all solar energy systems is the collection ofdust on the collecting surface. This problem has been the subject of anumber of studies because dust in the collecting aperture of any solarsystem, concentrating or not, will reduce its collecting efficiency overtime. The presence of snow and water are related but separate problemsthat need to be dealt with. Getting rid of dust often requires a sitevisit by maintenance personnel to remove the dust with soap and water.This in turn will require that the solar energy system be shut down toavoid possible injury or death due to the present of high voltage.

As noted, the tray 125 of the collector 100 preferably has a transparentcover 130 disposed on the open trough of the tray 125. To deal with dustand the like, a brush 126 depends from the tray 125 and is configured tocontact with the cover of an adjacent one of the solar collectors 100with the second rotation of the adjacent solar collectors 100.

In one embodiment, solar collectors 100 (FIGS. 3A-3C) are spaced onframe 80 (FIG. 2) such that seasonal drive system 160 (FIGS. 8C-1, 8C-2,and 8C-3) can rotate solar collectors 100 to an angle that brings brushassembly 126 into contact with cover glass 130 (FIG. 3B) on adjacentmodules. In one embodiment, seasonal drive system 160 (FIGS. 8C-1, 8C-2,and 8C-3) is capable of performing this in two positions substantial180° apart so that cover glass 130 of all collectors 100 is brought intocontact with brush assembly 126 of an adjacent collectors 100. The brushsystem herein disclosed can mitigate the effects of snow and wateraccumulation and can perform some mitigation on the collection of dust.

One source of concern is the possibility of slippage of any system usingwire cable repetitively. The environment for outdoor operation will besubstantially difficult. Both on a daily and a seasonal basis such asystem will be exposed to a far wider range of temperatures and otherphenomena such as high wind, freezing fog, and the like. Avoiding suchslippage requires careful planning in the design.

FIGS. 10A-10D present an alternative approach to the systems of FIGS.8A-8G that rely on a first approach to the reduction of slippage. FIGS.10A-1, 10A-2, and 10A-3 depicts side view 221, top view 222, and bottomview 223 as seen from above. Views 221, 222, and 223 comprise drivenpulleys 210 a-e, driver pulley 215, tensioner (idler) pulley 220,tensioner spring 225 and wire cable 205. The configuration of the systemof FIGS. 10A-1, 10A-2, and 10A-3 differs from that of FIGS. 8C-1, 8C-2,and 8C-3 in that drive pulleys 210B-210E are each configured with thedouble wrap arrangement disclosed in FIG. 8G. The operation is otherwisesimilar to that of the system of FIGS. 8C-1, 8C-2, and 8C-3.

FIGS. 10B, 10C and 10D use the double wrap configuration disclosed inFIG. 8G on driven pulleys 210 b-e and are otherwise similar to thesystems previously disclosed in FIGS. 8D, 8E and 8F. The systems ofFIGS. 10B, 10C, and 10D comprise wire cable 205, driven pulleys 210 a-e,driver pulley 215, and tensioner pulley 220. The system of FIG. 10Bfurther comprises inline tensioning spring 226. The system of FIG. 10Cfurther comprises tensioner spring 225 deployed opposite driver pulley215. FIG. 10D further comprises compression spring 227 and tensioningspring 228.

FIG. 11A depicts an alternative way for preventing slippage in a wirecable driving comprising a set of pulleys and a fastener (screw) toprevent slippage mechanically. This obviates the need for the doublewrap method described in FIGS. 10A-10D. Wire cable 242 is routed aroundeach drive pulley 240 by a single wrap. Wire cable 242 is secured todrive pulley 240 by fastener 241, in this case a screw. The fasteningposition is approximately 180° across from the point on driven pulley240 at which wire cable 241 falls when the solar concentrator is at itsnull position as defined in this application. The enables driven pulleyto drive almost a full ±180° which is sufficient to operate the solarconcentrator system of this application.

FIG. 11B depicts the special case of the last driven pulley 241 a. Inthe case of driven pulley 241 a wire cable 242 must make an extra halfturn because wire cable reverses direction when it departs pulley 241 ato return to driver pulley 245 (not shown). Wire cable 242 is routedfrom driven pulley 240 to last driven pulley 240 a. Wire cable 242 isfastened to driven pulleys 240 and 240 a by fastener 241.

FIG. 11C depicts a range of positions represented by arc 247 on lastdriven pulley 240 a at which fastener 241 could be placed and stillallow the same range of rotary motion found on all other driven pulleys(not shown). Point 249 a indicates a point at which the upper part ofwire cable 242 first comes in contact with driven pulley 240 a. Point249 b indicates a point at which the lower part of wire cable 242 comesin contact with last driven pulley 240 a.

FIG. 11D depicts a view of last driven pulley 240 a of FIG. 11C asviewed from the closest driven pulley 240. (not shown) Wire cable 242routing from driven pulley 240 comes in contact with last driven pulley240 a at point 249 a. Wire cable 242 then wraps behind last drivenpulley 240 a (dotted lines) and then in front of last driven pulley 240a (as 242 a) and then continues behind last driven pulley 240 a (as 242b) until it reaches point 249 b at which point wire cable 242 breakscontact with last driven pulley 240 a and is routed directly to driverpulley 245 (not shown).

FIG. 11E depicts the arrangement of driver pulley 245 and driven pulley240. Wire cable 241 is routed from below to driver pulley 245. Wirecable 242 is wrapped about driver pulley 245 by 1½ turns and then isrouted to driven pulley 240. Wire cable 242 is wrapped one time aboutdriven pulley 240 and then is routed to other driven pulleys (notshown). In one embodiment, wire cable 242 is fastened to driver pulley245 in a manner similar to that of FIGS. 11C and 11D. Wire cable 242 maybe fastened to driven pulley 240 by fastener 241.

As noted herein, several of the solar collector systems 50 can bearranged together in an array or matrix on a rooftop or the like. Forexample, FIGS. 12A and 12B depict two different orientations ofrectangular roof top 230 with solar concentrator systems 232 mountedthereon. Solar concentrator systems 232 are mounted parallel to thesides of roof top 230 to maximize the number of solar concentrators thatcan be mounted thereon. FIG. 12A depicts a first orientation wherein twosides of roof top 230 are oriented substantially along a truenorth-south line. The frame axis (daily axis) is also orientednorth-south. Because solar concentrators 232 are oriented with dailyaxes (not shown) parallel to north-south solar concentrator systems 232may be driven with a simplified set of drive calculations as will beshown. FIG. 12B depicts a second orientation wherein no sides ofrectangular roof top 230 are substantially parallel to north-south.Maximizing the number of solar concentrator systems 232 mounted on rooftop 230 requires that the daily axes (not shown) of solar concentratorsystems 232 not be parallel to north-south. To drive to the sun positionmore complex drive calculations are required.

FIG. 12C depicts one solar concentrator system 234 wherein the solarconcentrator system 234 is not mounted with its daily axis parallel tolocal ground plane 235. One end of the daily axis of solar concentratorsystem 234 is at distance i from ground plane 235 and the other end ofthe daily axis is at distance k from ground plane 235, wherein distancei and distance k have different values.

The National Oceanic and Atmospheric Administration Earth SystemResearch Laboratory Global Monitoring Division (NOAA/ESRL/GMD) haspublished a Solar Position Calculator spreadsheet available in at leasttwo formats that can be used to calculate the position of the sun basedon geographic location (latitude and longitude), local date, and timepast local midnight. The calculator is stated to be based on equationsfrom “Astronomical Algorithms” by Jean Meeus. One result yielded by theCalculator is an apparent position of the sun in terms of Solar Azimuthmeasure clockwise from true north and apparent Solar Elevation indegrees corrected for atmospheric refraction. This is an open sourcetool available to the general public that is used for a number ofapplications. In this application the Solar Azimuth and apparent SolarElevation are used as an input to another set of equations used tocalculate a position of the sun relative to the axes of solarconcentrator system 50 of FIG. 2. In this application the Solar Azimuthangle is referred to as “Theta AZ” and the apparent Solar Elevation(Altitude) angle is referred to as “Theta ALT”. The NOAA SolarCalculator may be found athttp://www.esrl.noaa.gov/gmd/grad/solcalc/NOAA_Solar_Calculations_day.xlsand instructions for its use may be found athttp://www.esrl.noaa.gov/gmd/grad/solcalc/calcdetails.html.

Referring to FIG. 2 and FIG. 7C, both daily drive motor 65 and seasonaldrive motor 104 of a solar concentrator system (50) may be steppermotors. Because, in a properly designed system, stepper motors can bedriven open loop to known angular positions, it is convenient to definea local coordinate system based on angles of rotation of frame 80 andconcentrator modules 55. Both coordinates are angles which may beexpressed in radians or degrees or in any other convenient system. Mostcomputing systems use radians for actual calculations but degrees aretypically easier to place in context. A spherical coordinate systembased on a daily drive and a seasonal drive is similar to thelatitude-longitude system commonly used to specify specific locations onthe earth. Similarly, the daily drive system are somewhat analogous tosolar azimuth and solar elevation in the NOAA position calculator. Thedaily drive system is analogous to an azimuth system and the seasonaldrive system is analogous to an inclination system. Daily drive motor 65of FIG. 2 and seasonal drive motor 104 of FIG. 7C provide the way toposition solar module 124 of FIG. 3C such that its optic axis (arrow c)is aligned to a desired set of angles in the coordinate system of thesolar concentrator system (not shown).

For the purposes of discussing the calculation and control of thedisclosed system, FIG. 13A depicts a coordinate system 233 based on theangular positions of a daily drive system after FIGS. 5A and 5B and aseasonal drive system after FIG. 8A. The convention for this applicationregarding the angles of orientation of the daily drive is that angles tothe east of the normal point are negative and angles to the west of thenormal point are positive. The convention for this application regardingthe angles of orientation of the seasonal drive is that angles north ofthe normal point are negative and angles south of the normal point arepositive. The following table presents the sign values for anglesrepresenting the position of a daily drive system after FIGS. 5A and 5Band the position of a seasonal drive system after FIG. 8A wherein thesign depends upon on which quadrant each of the two angles fall into.

Angle Sign Quadrant from FIG. 13A Daily System Seasonal System ′aPositive Negative ′b Negative Negative ′c Positive Positive ′d NegativePositive

A first assumption regarding the null coordinates is that the null pointfor the daily axis occurs at solar noon. At this time the optical axisis at its closest approach to vertical on the daily axis. The definitionof the null point for the seasonal drive system is presented in FIGS.13B and 13C. An assumption of this is that this is determined with thedaily axis at its null position. FIG. 13B comprises a first orientationof frame 234 wherein frame 234 is parallel to ground plane 235 at adistance i from ground plane 235 at all points. Optical axis 236 isperpendicular to the frame and is oriented parallel to a vertical axisperpendicular to ground plane 235. The coordinate for the daily axis isexpressed in degrees as Theta E-W and the coordinate for the seasonalsystem is expressed in degrees as Theta N-S, both with signs asindicated above.

FIG. 13C depicts a second orientation of frame 234 wherein the dailyaxis of frame 234 is not parallel to ground plane 235 and rather standsat a first distance i at a first end of the frame 234 end and at asecond distance k not equal to i at an end opposite to the first end offrame 234, wherein the measurement points for i and k are separated by adistance m. According to standard rules of trigonometry, optical axis236 in this case forms an angle n with a vertical axis 237 perpendicularto ground plane 235 wherein the angle n follows the following equation.

$n = {\arctan \frac{\left( {k - i} \right)}{m}}$

The use of the coordinate system of FIGS. 13A-13C requires a systematicapproach. In a first case a solar concentrator system 50 such as that ofFIG. 2 is assumed to be oriented with its daily axis system orientednorth south as in FIG. 12A and with its frame parallel to the localground plane as shown in FIG. 13B. The angle of the daily axis,hereinafter referred to as Theta E-W, is positive when facing west(solar afternoon) and negative when facing east (solar morning) in thisconfiguration. The angle of the seasonal axis, hereinafter referred toas Theta N-S, is positive when facing south from a line perpendicular tothe frame and negative when facing north of a line perpendicular to theframe.

The previously stated output of the NOAA Solar Position Calculator forSolar Azimuth (Theta AZ) is an angle measured clockwise from true northand the output for Solar Elevation is (Theta ALT) is degrees above thehorizon corrected for atmospheric refraction. In order to point a solarconcentrator system, it may be necessary to perform a transformationupon the output of the Solar Position Calculator from Theta AZ and ThetaALT to the coordinates of the solar concentrator system (Theta N-S andTheta E-W).

The following convention is used in the conversion equations:

Sign calc=|Theta AZ−180°|/(Theta AZ−180°)

-   -   Sign calc insures the correct angle sign results in the        conversion from Theta ALT and Theta AZ to Theta N-S and Theta        E-W.        The following conversion equation will transform Theta ALT and        Theta AZ to Theta N-S and Theta E-W.

${{Theta}\mspace{14mu} E\text{-}W} = {\left( {{90{^\circ}} + \left( {{Signcalc} \times {{Arctan}\left( {{Tan}\frac{{Theta}\mspace{14mu} {ALT}}{\cos \left( {{{Theta}\mspace{14mu} {AZ}} - {90{^\circ}}} \right)}} \right)}} \right)} \right) \times {Signcalc}}$Theta  N-S = Arccos((−sin (Theta  AZ − 90^(∘))) × Cos(Theta  ALT))

A program using these formulae must include error traps to avoiddivision by zero creating overflow conditions, as is well known in theart. One example of an overflow condition would occur when

To have the broadest possible range of installation options, it isoptimal to be able to orient a set of solar concentrators without arequirement to orient the daily axis to exactly true north. FIG. 12Bprovides an example of this. The previous equations are useful, but mustbe expanded to be useful in a more arbitrary installation.

The approach taken to this problem is to develop a second set ofequations useful to take Theta AZ and Theta ALT as inputs and derivefrom them a virtual Theta AZ and Theta ALT which can be used as Theta2and Theta1 in the above equations, which in turn are useful to calculateTheta E-W and Theta N-S.

One convention in this application is to continue the use of daily driveand daily axis and seasonal drive and seasonal axis.

FIG. 14A presents convention 240 for angles related to tilt of the dailyaxis relative to local ground plane 243. Line 241 depicts a situation inwhich the south end of the daily axis is higher relative to the groundplane than the north end. The angle that line 241 forms with the groundplane is deemed positive. Line 242 depicts a situation in which thesouth end of the daily axis is close to the ground than the north end.The angle that line 242 forms with ground plane 243 is by conventiondeemed negative.

FIG. 14B presents convention 245 for angles related to the orientationof the daily axis relative to true north. Angle 246 presents a casewherein the daily axis is oriented to the west of true north, whichangle is a negative value. Angle 247 presents a case wherein the dailyaxis is oriented to the east of true north, which angle is a positivevalue. Those of ordinary skill in the art will recognize that an angleof +200° is identical to an angle of −160°.

The conversion method of the present application involves applying anumber of transforms in the form of a series of equations with a numberof intermediate data values needed to arrive at a final value for theinputs to the preceding equations.

The NOAA Solar Calculator provides Theta AZ and Theta ALT according totime of day. Preset data values include latitude and longitude as thesewill not vary once a system is installed at a given location. Thissystem of coordinates is essentially a spherical coordinate system. Therange to the sun is approximately 93,000,000 miles, but the number canbe normalized to one (1) without comprising any calculations as will beshown in the texts for FIG. 15A.

The entry data for the process is to obtain the time of day in order toretrieve the values for Theta AZ and Theta ALT. Once these are obtainedthen the transform process can begin. Preset values in the transformprocess are those values that do not change once a system is installedat a given location that have not already been taken into account withinthe NOAA Solar Calculator. The two values of main interest are the dailyaxis tilt relative to the local ground plane as described for FIG. 14Aand the daily axis orientation relative to true north as described forFIG. 14B.

FIG. 14C depicts schematic 250 of the Y-Z plane of a solar concentratorsystem after the present application. As previously stated, theeast-west axis (Y-axis 252) is a plane parallel to the local groundplane. It may be considered to be substantially pointed east-west but isnot necessarily precisely east-west for reasons previously stated.Z-axis 251 is the plane perpendicular to the frame (not shown) of thesolar concentrator system when the ends of the frame are substantiallyparallel to the local ground plane. Angles falling west of Z-axis 251are positive and angles falling east of Z-axis 251 are negative.Semicircle 354 represents the arc formed by vector 253 as it movesthrough a day. Vector 253 represents a projection of a unity pointingvector perpendicular to the frame (not shown) of a solar concentratorsystem onto the Y-Z plane defined by Y-axis 252 and Z-axis 251.

As will be appreciated, a number of mathematical calculations arerequired to translate the output of the NOAA Solar Calculator into aform that can be used by a solar concentrator system according to thepresent disclosure. The steps can be accomplished using mathematicalformula disclosed in various documents or sources, such as Wikipedia.One source is “C.R.C. Mathematical Tables, Thirteenth Edition,” 1964,published by The Chemical Rubber Company, Robert C. Weast, Editor. Thesection on analytical geometry on pp. 446-450 is particularly useful.The coordinate system of the solar concentrator system is alsospherical. The angle of each solar concentrator unit is equivalent toelevation and the angle of the frame is equivalent to azimuth. Thus thepresent calculations convert from one coordinate system to anothercoordinate system of the same type but with differing orientation ofaxes.

FIG. 15A presents a 3D representation of the method used to calculatethe position of the sun in the coordinate system of the solarconcentrator system. The coordinate system of FIG. 15A comprises astandard Cartesian three dimensional grid with mutually perpendicularaxes annotated as X-axis 261, Y-axis 270 and Z-axis 262. Sphericalcoordinate outputs are depicted in the drawing. Vector 276 of length r0terminates at position A with coordinates X1, Y1 (not shown), and Z1.Vector 276 represents the angle of a solar axis defined by Theta AZ andTheta. ALT from the NOAA Solar Concentrator translated into the XYZcoordinate system. Projection 263 of vector 276 onto the XZ planedefined by X-axis 261 and Z-axis 262 falls at point B with coordinatesX1 and Z1. Vector 263 drawn from the origin of the grid system to pointB forms angle θ_(3XZ) with Z-axis 262. Vector 263 terminating at point Bis rotated by an angle θ_(2ZX) which is defined to be the value of ThetaALT Offset. Rotated vector 264 terminates at point C. All rotation takesplace in the XZ plane about Y-axis 270. Since θ_(1XZ) and θ_(2XZ) fallin the same plane and are adjacent angles with the same vertex, theangle θ_(3XZ) between Z-axis 262 and rotated vector 264 is equal toθ_(1XZ)+θ_(2XZ). Based on the length of rotated vector 264 and angleθ_(3XZ) it is possible to calculate new coordinates X2 and Z2. Thelength r1 of vector 263 is equal to √(X1²+Z1²). Since vector 263 isrotated to point C in the XZ plane axiomatically it remains at lengthr1. Thus r1=√(X2²+Z2²) and therefore √(X1²+Z1²)=√(X2²+Z2²). Anotherconsequence of the equality of √(X1²+Z1²) and √(X2²+Z2²) is that Y1=Y2.This follows because r0=√(X1²+Y1²+Z1²) and r0=√(X2²+Y2²+Z2²). Since itis established that √(X1²+Z1²)=√(X2²+Z2²), by substitution√(X1²+Y1²+Z1²)=√(X1²+Y2²+Z1²). Commonplace mathematical operationsreduce this to Y1=Y2 (hereinafter Y, Y1 and Y2 are understood to havethe same value for a rotation based on the same input Theta AZ and ThetaALT as indicated by distance line 280). This is also logical since allrotation is around the Y-axis. Since X2, Y (Y2), and Z2 are known, theycan be used to calculate Theta AZ Out and Theta ALT Out which arerepresented by point D at the end of vector 272.

Angle Theta AZ2 X1 is defined as the included angle between projection277 of vector 276 in the XY-plane and Y-axis 270. Angle Theta AZ2 X2(not shown) is defined as the included angle between projection 279 ofvector 272 and Y-axis 270. Theta AZ2 Delta is defined as the anglesubtended by projection 277 of vector 276 onto the XY plane andprojection 279 of vector 272 onto the XY plane. By inspection Theta2X1−Theta2 X2=Theta AZ2 Delta Theta AZ is the angle subtended byprojection 277 of vector 276 and the north end of the daily axis (notshown). The daily axis is nominally the X-axis. Theta AZ Out is theangle subtended by projection 279 of vector 272 and the north end of thedaily axis (not shown). Theta AZ Out is equal to Theta AZ+Theta AZ2Delta with some adjustments disclosed in conjunction with FIG. 15C.Theta ALT Out is the arcsine of Z2√(X2²+Y²+Z2²).

FIG. 15B depicts a view of the XZ plane of FIG. 15A as viewed alongY-axis 270. Vector 263 represents an angular position of a solarconcentrator module (not shown) wherein the solar concentrator module ispart of a solar concentrator system after FIG. 13B wherein the dailyaxis is substantially parallel to the local ground plane. Vector 263forms angle θ_(1xz) with Z-axis vector 262. Vector 264 represents theposition vector after the south end of a daily axle (not shown) situatedin the Northern hemisphere is raised such that the south end of thedaily axle is higher than the north end of the daily axle relative tothe local ground plane (not shown) in a manner analogous to thatdisclosed in FIG. 13C. In practice the degree of incline of X-axis 261relative to the ground plane is equal to angle θ_(2XZ). The angle formedbetween vector 263 and vector 264 is θ_(2XZ), making the total angle 3XZ between rotated vector 263 and Z-axis 262 equal to θ_(1XZ)+θ_(2XZ).Vector 263 projects a distance X₁ onto X-axis 261 and a distance Z₁ ontoZ-axis 262. Vector 264 similarly projects a distance X₂ onto X-axis 261and a distance Z₂ onto Z-axis 262. X2 and Z2 may be considered thecoordinates of vector 264 in Cartesian coordinate form.

Stated in other terms, the following equations apply.

X1=r1*sin θ_(1XZ)

Z1=r1*cos θ_(1XZ)

X2=r1*sin θ_(3XZ)

Z2=r1*cos θ_(3XZ)

For the present application, only a simple example requiring acoordinate transform is presented. The case in point is the examplewhere the daily axis is not aligned to the north-south axis but ratheris rotated to some arbitrary angle. The example does not involve tilt ofthe daily axis along the north-south axis as described in FIG. 14A andthe rotation of the frame about the daily axis includes a point at whichthe frame is substantial parallel to the local horizontal plane.

FIG. 15C presents a table comprising a series of calculations thataccept latitude, longitude local time and angle Theta AZ Offset, whichis the angle between the daily axis of the solar concentrator system andthe north-south axis. The table yields Theta N-S and Theta E-W. ThetaE-W and Theta N-S are a position of the sun in the coordinate system ofthe solar concentrator system as described in FIGS. 14A, 14B and 14C.Theta E-W and Theta N-S are then adapted to be used as inputs in thepreviously described equations useful to position the daily axis andseasonal axis (not shown) of a solar concentrator system so that theoptical axes of the solar concentrator modules are substantiallyoriented at the sun. The calculations may be implemented in commerciallyavailable spreadsheets, such as LibreOffice Calc or Microsoft OfficeExcel. The calculation may alternatively be implemented in higher orderprogramming languages such as C, C++, C #, Pascal, and many others.

The table of FIG. 15C is organized into 7 major blocks noted by lettersA through G. The steps within all blocks are number sequentially as 1through 22. Those of skill in the art will recognize that many of thesteps can be taken in orders other than the presented order withoutchanging the output of the set of calculations. Numbering in thefollowing paragraphs shall refer to FIGS. 15A and 15B.

Block A, steps 1 and 2, describes the information to be collected forthe initial calculations of the NOAA Sun Position Calculator. Latitudeand longitude are most typically described in degrees, minutes andseconds East or West and North or South, although a program may beconfigured by its programmer to accept alternative forms such as degreesincluding minutes and seconds converted to a decimal form. Latitude andlongitude do not vary with time unless the solar concentrator systemitself is moved for any reason, such as maintenance. Real time clock maysimilarly be expressed in hours, minutes and seconds or alternative ashours and decimal fractions thereof.

Block B, steps 3 and 4, present the data output of the NOAA SolarPosition Calculator. Theta AZ is determined clockwise from true northand Theta Alt is determined as up from the local horizontal plane. Bothmay be presented in degrees, degrees, second and minutes or in radiansor in arbitrary units.

Block C, step 5, presents the value Theta AZ Offset, which is the anglebetween the north-south axis of the earth and the orientation of thedaily axis of the solar concentrator system. This angle does not varywith time unless the solar concentrator system itself is moved for anyreason, such as maintenance. The angle may be determined through the useof ordinary surveying equipment or the like.

Block D, step 6, presents a step which subtracts Theta AZ Offset fromstep 5 from Theta AZ to yield Theta2 AZ, which is the calculatedposition of the sun in an intermediate coordinate system similar to theoutputs of the NOAA Sun Position Calculator but with the azimuth angleshifted from relative to true north to the orientation relative to theangle of the daily axis of the solar concentrator system. This is aninterim set of calculations

Block E, steps 7-10, performs the full transformation from theintermediate coordinates of step 6 to a set of coordinates in thecoordinate system of the solar concentrator system.

Step 7 calculates a sign value multiplier normalized to one or minus oneso that it only affects the sign of values multiplied by it. The purposeis to insure that the final result of the calculations of the tableconforms to the sign conventions disclosed in FIGS. 14A through 14C.

Step 8 calculates an intermediate value comprising part of the equationfor Theta E-W disclosed in the part of the specification preceding thetext regarding FIG. 14A.

Step 9 calculates the value of Theta E-W using the intermediate valuefor Step 8 together with the remainder of the equation for Theta E-Wpreviously noted. Theta E-W is the angle of the daily axis as previouslydescribed.

Step 10 calculates the value of Theta N-S implementing fully theequation previously disclosed. Theta N-S is the angle of the seasonalaxis which positions each of the solar concentrator module

Block F, step 11, describes the method for converting coordinates basedon the two orthogonal axes of a solar concentrator system into commandsuseful to drive a stepper motor or similar device from a first positionto a second position.

Thus, a full tracking solution, is provided for a solar concentratorsystem comprising a two angular movement systems operative to track thesun in both azimuth and altitude. The solar concentrator system can beoperated open loop if necessary, or it may be operated with closed looptracking with the use of the tracking solution only for initialoperation to acquire the sun. An additional alternative is to operateopen loop with periodic closed loop tracking updates to insure continuedoptimal operation. The trigger for an update may be based on time of dayor alternatively on reduction in output providing a provision is madefor shading by clouds or other obstacles. The open-loop tracking makesuse of trigonometric calculations to convert from Theta AZ and Theta ALTfrom the NOAA Solar Calculator to a coordinate system of the solarconcentrator system based on control of two stepper motors. The solarconcentrator makes use of pulleys connect by wire cable to a driverpulley attached to a stepper motor to insure all solar concentratormodules move in concert.

One aspect of the solar concentrator system 50 is a need to achieve thegreatest possible performance at the lowest possible cost. Whileperformance is easily quantified, cost often must include items that donot contribute directly to performance such as the value of the surfaceon which the solar concentrator system must be mounted. For this reason,the performance of the solar concentrator system 50 is normally at somesort of premium.

One problem with the solar concentrator system 50 using reflectiveconcentrator mirrors is that the materials used to form the reflectivesurface often have efficiency that varies within the spectrum ofinterest. FIG. 16A presents the reflectance spectrum for a mirror formedby coating a planar surface with aluminum. The reflectance for thevisible light spectrum is roughly in the 90-92% range. At 800 nanometers(nm) the reflectance reaches a low point of 85.5% and for longerwavelengths beyond 800 nm the reflectance gradually rises to 97%. Animprovement to reflectance at all wavelengths below 1100 nm would beuseful.

An additional problem is that metals used for mirrors such as aluminumand silver are prone to oxidation which will further reduce itsreflectivity. The problem of oxidation can be solved through the use ofdeposited coatings that protect the metal surface from oxidation, whilethe reflectance problem requires a different type of solution.

One solution to both problems may be found in the use of commerciallyavailable films that can be applied to a suitably formed mirrorsubstrate. An example is the Cool Mirror Film 330 available from 3M™.These films include a reflective metal layer that is encapsulated sothat oxidation of the metal layer does not occur. The film also includesdielectric layers above the metal layer that are designed to reflectlight in the wavelength ranges where the metal reflectance is deficientand to not interfere with other wavelength ranges.

This property of dielectric mirrors is known. US 2009/0283133, forexample, disclose a variety of possible dielectric mirror performancesthat are tuned to different parts of a spectrum of solar radiation. Itis obvious that as part of the tuning the reflective properties of theunderlying substrate can be taken into account.

Evaluation of the commercially available films has resulted in theunderstanding that the stiffness of these materials is relatively high.The inverse property to stiffness is compliance and materials of a lowcompliance are stiffer than materials of a high compliance. These filmscan easily be attached to a planar surface or to a trough mirror withgain in only one axis. Attaching the same film to the surface of amirror or substrate with curvature in more than one axis, such as thatof FIG. 16B, represents a more serious challenge. FIG. 16B depicts aconcave mirror substrate 301 with curvature in two orthogonal axes. Useof a pressure sensitive adhesive on the surface of the planar substratethat is adjacent to the curved substrate is a common and accepted wayfor attaching a highly compliant material to a more rigid one or ofattaching two thin materials. This is well known in the art.

FIG. 16C illustrates the problem. Mirror substrate 301 represents aconcave surface with curvature in two orthogonal axes while film 302 isplanar. A planar substrate can be made to curve in one axis with theunderstanding that the stiffness of the material may limit the radius ofcurvature to a minimum number. Fixing a planar substrate to a concavesubstrate with curvature in two axes necessarily requires deformation ofthe planar substrate.

To overcome these difficulties, the present disclosure teaches a processto apply a planar film 302 to a substrate 301 having a concave surfacewith curvature along two axes. As briefly discussed here, a planar film302, such as shown in FIG. 16D, is provided that has a plurality offirst slots 303 a defined radially from an outer edge toward a centralportion of the planar film 302. The planar film 302 is also providedwith a plurality of second slots 303 b defined in the outer edge of theplanar film 302. The planar film 302 is oriented adjacent the concavesurface of the substrate 301 by supporting the outer edge of the planarfilm 302 with the second slots 303 b. The planar film 302 is movedagainst the concave surface by: applying force against the planar film302 from an applicator (e.g., 325 a: FIG. 16F, 403: FIG. 16H) having adeformable surface, affixing the central portion of the planar film 302first to the concave surface with the applied force of the application,and affixing progressively radial portions about the central portion ofthe planar film 302 to the concave surface by deforming the deformablesurface of the applicator with the applied force.

Looking at the application techniques in more detail, FIG. 16D depicts apattern of slots 303 a-b that may be cut into a planar film 302. Thisapproach is useful when the stiffness of a planar film is high enough tomake it difficult to deform. By removing material as shown it has beendemonstrated experimentally that a planar film 302 with high stiffnesscan be used to form a reflective surface on the surface of a concavemirror substrate 301 with curvature in two axes. Patterns of slots 303a-b other than the pattern shown have been tested and proven to work.

FIG. 16E depicts vacu-form (vacuum forming) device 305. Vacu-form device305 comprises vacuum housing 309, evacuation port 306, platen 308, mold311, toggle clamps 307 and planar substrate 310 and a vacuum pump (notshown). The vacuum pump is operative to reduce the pressure of air orany other gas within the vacuum housing. In one embodiment, a heaterassembly (not shown) is mounted above planar substrate 310 to heat andthereby soften the material to add thermo-forming capability. In oneembodiment, a vacuum seal membrane (not shown) is placed above planarsubstrate 310 to reduce the surface area of planar substrate 310required to the minimum necessary. The vacuum seal membrane pushes downon planar substrate 310 forcing it to conform to the shape of mold 311.In an embodiment mold 311 is a mirror substrate similar to mirrorsubstrate 301.

A vacu-forming process can be adapted to attach a planar film to asubstrate. This differs in that the planar film should adhere to thesurface of the substrate rather than releasing after pressure is removedas is typical of a mold process. A limitation of a vacu-forming approachis that the pressure that is applied to planar substrate 310, whetherdirectly or by a vacuum seal membrane, is limited to atmosphericpressure, nominally 14.696 pounds per square inch or 101.325 kPa at sealevel. In some instances, it may be desirable to be able to apply agreater level of pressure. Another desirable characteristic of a methodof attaching a planar film to a substrate with curvature in two axes isthat the film should adhere to the surface without forming air bubblesor other inclusions that may lead to a tarnishing of the reflectivemetal layer due to oxidation. While it is possible to performmanufacturing in a relatively inert atmosphere such as nitrogen, it isdesirable to find a less complex way to do this.

The point at which to apply additional pressure is a position wherein,the additional pressure reinforces the movement of the vacuum sealmembrane of FIG. 16E in pressing planar substrate 310 onto mold 311. Thedistribution of the additional pressure is a matter for consideration. Aparticular mold configuration may dictate that a particular approach ismore suitable than another. A surface configuration of interest is thesituation of a concave surface with curvature in two axes such as thatshown in FIG. 16B.

FIG. 16F discloses a solution to these problems. Vacu-forming device 315comprises vacuum housing 316, evacuation port 317, platen 318, shims319, and deformable membrane 323. Shims 319 act to hold mirror substrate320 steady on platen 318. Mirror substrate 320 comprises a concavesurface with curvature on two axes, wherein interior curve 322 comprisesa maximum depth curve, and interior curve 321 comprises a set of edgesof the mirror substrate 320 parallel to interior curve representinglocal maxima relative to interior surface 322. Similar curves exist inthe orthogonal view (not shown). An applicator or ball 325 a represents,in one embodiment, a sphere with a gas sealed within, acting as apressure medium, selected such that its radius of curvature (not shown)is less than that of interior curve 322 and is also less than that ofthe interior curve orthogonal (not shown) to interior curve 322. Inanother embodiment, the sphere is a solid sphere with no internal gasfabricated from a suitably compliant material. Establishing thisrequirement for radius of curvature insures that the first point ofcontact between mirror substrate 320 and ball 325 a is at a pointconsidered to be the bottom of the central curve 322 of mirror substrate320 as depicted.

In one embodiment, evacuation port 317 is connected to a vacuum pump(not shown) by suitable hoses (not shown). In one embodiment, evacuationport 317 is not connected to a vacuum pump and acts as a vent port.

Using a pressurized gas within ball 325 a assists in insuring that thecompliance of ball 325 a is limited. The actual internal pressure may besomewhat above or even equal to atmospheric pressure, but the surface ofball 325 a is a curvilinear sealed entity into which no substantialamount of gas may be introduced or taken out during routine operation.When pressed by force 326 from a position opposite to the position ofmirror substrate 320, ball 325 a is deformed into non-spherical ball 325b. Ball 325 a and ball 325 b represent the same physical ball for theapplicator in different states of deformation and are not to beconstrued as separate items. Force 326 may be applied by a mechanicalelement similar in concept to the movement of a drill press chuck (notshown) and spindle (not shown) without the rotation normally associatedwith the spindle. The central axis of the chuck and spindle is aimed tolowest point 327 of mirror substrate 320 such that the first point ofcontact between ball 325 a and mirror substrate 320 is the lowest point327. Mirror substrate 320 may be fixed in a suitable locating with a jig(not shown) or another structure with similar function.

As further pressure is applied, ball 325 a conforms to the curvature ofconcave mirror substrate 320 in both axes. The point of contact withconcave mirror substrate 320 will spread from lowest point 327 asdepicted to positions further out on all sides. This spread of the pointof contact insures that reflective film 324 can be applied to mirrorsubstrate 320 without the development of air bubbles or otherinclusions. Those of skill in the art will recognize that the approachdepicted in FIG. 16F need not necessarily require the use of avacu-forming device such as that shown in FIG. 16E, although it may benecessary that the assembly have a vent port to insure that no internalpressure develops during the application of a planar substrate. Theprocess described above need not be limited to mirror surfaces.

One issue with the use of spherical ball 325 a for the applicator isthat the radius of ball 325 a limits the surface area over which addedpressure can be applied. This is inherent in this choice of pressuremedium. The exact area is dependent on the compliance of ball 325 a.Ball 325 a must have sufficient stiffness to be able to push reflectivefilm 324 onto the surface of mirror substrate 320 and also must have alow enough stiffness to conform to the surface of concave mirrorsubstrate 320 under pressure. These requirements are somewhat inconflict but applicant has demonstrated this in experimentation so it isproven that both requirements can be satisfied through selection of asuitable material for ball 325 a. In one embodiment, ball 325 a isfilled with air. As pressure is applied to ball 325 a to deform it asshown in ball 325 b, the volume of ball 325 b shrinks somewhat relativeto ball 325 a and the pneumatic air pressure within ball 325 b rises.This in turn will increase the stiffness of ball 325 b.

FIG. 16G depicts one way of overcoming this limitation. Curved surface328 are shown to represent the curvature of a mirror substrate (notshown), and curved surface 329 are shown to represent the curvature of acurvilinear pressure medium (not shown) used to press a film similar toreflective film 324 of FIG. 16F onto the surface of the substrate. Theuse of a curvilinear pressure medium pressure medium still requires thatthe local radius of curvature of the pressure medium is less that theradius of curvature of the substrate. By inspection, the radius ofcurvature for curved surface 328 is clearly greater than that ofcurvature 329 of the pressure medium. In one embodiment, the substrateis a mirror substrate.

FIGS. 16H and 16I present an alternative assembly device 400 capable offixing a planar film to a concave surface with curvature in two axes.Assembly device 400 comprises top plate 401 with inlet aperture 402affixed thereto, pressure system frame 405 with evacuation port 406affixed thereto, bottom plate 407 with jig components 408 affixedthereto, substantially impermeable membrane or applicator 403, and filmholding fixture 409. Together, pressure frame 405 and bottom plate 407comprise a vacuum housing similar to vacuum housing 316 of FIG. 16F. Inone embodiment, evacuation port 406 is affixed to bottom plate 407.Concave substrate 410 is held in placed by jig components 408 such thatit is oriented in a position with its concave surface facing membrane403. (Substrate 301 of FIG. 16B depicts an alternative view of concavesubstrate 410.)

Film 404 is positioned between concave substrate 410 and the membrane orapplicator 403. Film 404 is held in place substantially parallel to topplate 401 by holding fixture 409. Ideally, holding fixture 409 is afriction restraining device able to release film 404 when sufficientpressure is applied perpendicular to the plane of film 404. In oneconfiguration, a set of binder clips may be used to provide thisfriction in conjunction with a cross bar Other solutions are available.

Membrane 403 as the applicator is a substantially impermeable membraneselected to have substantially compliance and elasticity. Membrane 403may be mounted directly to top plate 401 at a position near theinterface between top plate 401 and pressure system frame 405.Alternatively, membrane 403 may form part of a gasket between top plate401 and pressure system frame 405 or may be affixed to the interiorwalls of pressure system frame 405. Membrane 403 acts to divide theinterior of assembly device 400 into two chambers, an upper chamber, anda lower chamber. The upper and lower chambers are of variable size.Inlet aperture 402 is connected to a pressure pump (not shown) by hoses(not shown) such that a gas under pressure may be introduced throughinlet aperture 402 into the upper chamber and thereby cause membrane 403to expand inside the interior chamber of assembly device 401.

Evacuation port 406 is connected to a vacuum pump (not shown) by hoses(not shown) wherein the vacuum pump is operative to pump gas from thelower chamber of assembly device 400 and thereby cause membrane 403 toexpand into the lower chamber of the interior of assembly device 400.The hoses connected to the upper and lower chambers of the interior ofassembly device may be configured with relief valves operative torestore the upper chamber and the lower chamber to the pressure of thegas surrounding the exterior of assembly device 400. In one embodiment,evacuation port 406 is not connected to a vacuum pump and acts as a ventport.

When a vacuum pump acts alone to evacuate the lower chamber and nopressure is added to the upper chamber, the limit on pressure that canbe applied is the pressure of a standard atmosphere, assuming that allgas is evacuated from the lower chamber. This was demonstratedexperimentally to be insufficient for the application of a film withsubstantially stiffness to a concave substrate with curvature in twoaxes. In one embodiment, a vacuum pump (not shown) is operative toremove gas from the lower chamber of the interior of assembly device 400and a pressure pump (not shown) is operative to apply pneumatic pressureto the upper chamber of the interior of assembly device 400substantially simultaneously.

In some instances, it may be advantageous to operate with film 404 at anelevated temperature to reduce its stiffness. In one embodiment, aheating element (not shown) is included in film holding fixture 409 totransfer heat to film 404. In one embodiment, the gas within theinterior of assembly device 400 is pre-heated. In one embodiment, thegas introduced through inlet aperture 402 is preheated.

While this has been described by way of example, and in terms ofembodiments, it is to be understood that the present disclosure is notlimited to the disclosed embodiments. To the contrary it is intended tocover various modifications and similar arrangements that would beapparent to those skilled in the art. Therefore, the scope of theappended claims should be accorded the widest possible interpretation soas to encompass all such modifications and similar arrangements.

I claim:
 1. A system for use on a surface to collect solar energy fromthe sun, the system comprising: a stand supportable on the surface andhaving first opposing ends, the first opposing having first rotationpoints; a frame having second opposing ends with second rotationalpoints, the second rotational points rotatably disposed on the firstrotational points of the stand, the frame rotatable on the first andsecond rotational points about a first axis of rotation; a first drivedisposed on the stand and operable to provide first rotation; a hoopdisposed on the frame and defining a curvature about the first axis; atleast one first cable connected between the hoop and the first drive androtating the frame about the first axis with the hoop in response to thefirst rotation of the first drive; and solar collector means affixed tothe frame.
 2. The system of claim 1, wherein the at least one firstcable comprises a pre-stretched wire cable.
 3. The system of claim 1,wherein the first drive comprises an encoder, a stepper motor, a linearactuator, a DC motor, an AC motor, a brushless motor, or a brushedmotor.
 4. The system of claim 1, wherein the first drive comprises adrive pulley having the at least one first cable disposed thereon andbeing rotatable with the first rotation by the first drive.
 5. Thesystem of claim 4, wherein the drive pulley defines a helical groovethereabout in which the at least one first cable at least partiallywraps.
 6. The system of claim 4, wherein the at least one fir cablecomprises: A first cable segment having one end affixed to the drivepulley and having an opposing end affixed to the hoop on one sidethereof; and A second cable segment having one end affixed to the drivepulley and having an opposing end affixed to the hoop on an opposingside thereof.
 7. The system of claim 1, further comprising a firsttensioner tensioning the at least one first cable.
 8. The system ofclaim 7, wherein the first tensioner comprises a turnbuckle disposed inline with the at least one first cable, an idler pulley, a biasingelement, a catch biased to grip in tension and release in slack, or acombination thereof.
 9. The system of claim 1, further comprising acontrol system operating the first drive to repeatedly orient a lineperpendicular to the frame to fall into a plane defined by the firstaxis of orientation and the sun.
 10. The system of claim 1, wherein thefirst axis of orientation is configured to orient substantially parallelto a north-south cardinal direction.
 11. The system of claim 1, whereinthe system further comprises at least one other stand having at leastone other frame, the stands positioned end-to-end, the first driveoperable to rotate the frames in tandem.
 12. The system of claim 1,wherein the solar collector comprises at least one solar photovoltaicdisposed on the frame.